Method For Monitoring A Process For Powder-Bed Based Additive Manufacturing Of A Component And Such A System

ABSTRACT

A gas diffusion electrode and electrolysis cells containing gas diffusion electrodes are provided. The gas diffusion electrodes include a copper-containing carrier, and first and second layers. The first layer comprising at least copper and at least one binder having hydrophilic and hydrophobic pores. The second layer comprising copper and at least one binder. The second layer present atop the carrier and the first layer atop the second layer, wherein the content of binder in the first layer is less than the binder in the second layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Application of InternationalApplication No. PCT/EP2016/067165 filed Jul. 19, 2016, which designatesthe United States of America, and claims priority to DE Application No.10 2015 215 309.6 filed Aug. 11, 2015, the contents of which are herebyincorporated by reference in their entirety.

FIELD OF INVENTION

The present invention relates to a gas diffusion electrode preferablycomprising a copper-containing carrier and a first layer comprising atleast copper and at least one binder and a second layer. The (first)layer comprises hydrophilic and hydrophobic pores and/or channels. Thesecond layer comprising copper and at least one binder, wherein thesecond layer is present atop the carrier and the first layer atop thesecond layer, wherein the content of binder in the first layer is lessthan in the second layer. The present invention also relates to aprocess for producing a gas diffusion electrode and to an electrolysiscell comprising a gas diffusion electrode.

BACKGROUND OF THE INVENTION

The combustion of fossil fuels currently supplies about 80% of globalenergy demand. In 2011, these combustion processes emit about 34 032.7million metric tons of carbon dioxide (CO₂) globally into theatmosphere. This release is the simplest way of disposing of largevolumes of CO₂ as well (large brown coal power plants exceeding 50 000 tper day).

Discussion about the adverse effects of the greenhouse gas CO₂ on theclimate has led to consideration of reutilization of CO₂. Inthermodynamic terms, CO₂ is at a very low level and can therefore bereduced again to usable products only with difficulty.

In nature, CO₂ is converted to carbohydrates by photosynthesis. Thisprocess, which is divided up into many component steps over time andspatially at the molecular level, is reproducible on the industrialscale only with difficulty. The more efficient route at present comparedto pure photocatalysis is the electrochemical reduction of the CO₂. Asin the case of photosynthesis, in this process, CO₂ is converted to ahigher-energy product (such as CO, CH₄, C₂H₄, C1-C4 alcohols etc.) withsupply of electrical energy which is preferably obtained from renewableenergy sources such as wind or sun. The amount of energy required inthis reduction corresponds ideally to the combustion energy of the fueland should only come from renewable sources or utilize electricity thatcannot be accepted from the grid at that moment. However, overproductionof renewable energies is not continuously available, but at present onlyat periods of strong insolation and/or wind. However, this state ofaffairs will further intensify in the near future with the furtherrollout of renewable energy or will level out since the installationswill be at different sites.

Not until the 1970s was there an increased level of systematic studiesof the electrochemical reduction of CO₂. In spite of many efforts, ithas not been possible to date to develop an electrochemical system withwhich CO₂ could be reduced with long-term stability and in anenergetically favorable manner to competitive energy sources withsufficiently high current density and acceptable yield. Owing to theincreasing scarcity of fossil raw material and fuel resources and thevolatile availability of renewable energy sources, research in CO₂reduction is moving into the focus of interest to an ever greaterdegree.

The electrochemical reduction of CO₂ to hydrocarbons, especially to thevaluable chemical raw material C₂H₄ (˜€1000/t) has been described in theliterature since the 1990s. There has been a significant rise inresearch activities over the last few years because the availability ofexcess electrical energy from non-fossil generation sources such assolar or wind is making the storage/utilization of this energy seemviable from an economic point of view.

For electrolysis of CO₂, in general, metals are used as catalysts, someof which are shown by way of example in table 1, taken from Y. Hori,Electrochemical CO₂ reduction on metal electrodes, in: C. Vayenas, etal. (eds.), Modern Aspects of Electrochemistry, Springer, New York,2008, pp. 89-189.

Table 1 shows the typical Faraday efficiencies (FE) over various metalcathodes. For example, CO₂ is reduced virtually exclusively to CO overAg, Au, Zn, and to some degree over Pd, Ga, whereas a multitude ofhydrocarbons are observed as reduction products over copper. As well aspure metals, metal alloys and also mixtures of metal andco-catalytically active metal oxide are also of interest, since thesecan increase the selectivity for a particular hydrocarbon. However, theprior art in this regard is not yet very developed.

TABLE 1 Faraday efficiencies for carbon dioxide over various metalelectrodes Electrode CH₄ C₂H₄ C₂H₅OH C₃H₇OH CO HCOO⁻ H₂ Total Cu 33.325.5 5.7 3.0 1.3 9.4 20.5 103.5 Au 0.0 0.0 0.0 0.0 87.1 0.7 10.2 98.0 Ag0.0 0.0 0.0 0.0 81.5 0.8 12.4 94.6 Zn 0.0 0.0 0.0 0.0 79.4 6.1 9.9 95.4Pd 2.9 0.0 0.0 0.0 28.3 2.8 26.2 60.2 Ga 0.0 0.0 0.0 0.0 23.2 0.0 79.0102.0 Pb 0.0 0.0 0.0 0.0 0.0 97.4 5.0 102.4 Hg 0.0 0.0 0.0 0.0 0.0 99.50.0 99.5 In 0.0 0.0 0.0 0.0 2.1 94.9 3.3 100.3 Sn 0.0 0.0 0.0 0.0 7.188.4 4.6 100.1 Cd 1.3 0.0 0.0 0.0 13.9 78.4 9.4 103.0 Tl 0.0 0.0 0.0 0.00.0 95.1 6.2 101.3 Ni 1.8 0.1 0.0 0.0 0.0 1.4 88.9 92.4 Fe 0.0 0.0 0.00.0 0.0 0.0 94.8 94.8 Pt 0.0 0.0 0.0 0.0 0.0 0.1 95.7 95.8 Ti 0.0 0.00.0 0.0 0.0 0.0 99.7 99.7

The following reaction equations show, by way of example, reactions atan anode and at a cathode for reduction over a copper cathode.

Of particular interest here is the formation of valuable ethylene.Reductions over other metals are analogous to these.

2CO₂+12e ⁻+12H⁺→C₂H₄+4H₂O  Cathode:

6H₂O→3O₂+12H⁺+12e ⁻  Anode:

2CO₂+2H₂O→C₂H₄+3O₂  Overall equation:

The individual electrode equations show that very complex processes thathave not been elucidated in detail to date are proceeding here with, forexample, CO or formate intermediates. For each of these intermediates, aparticularly preferred position at and/or on the copper cathodes shouldbe necessary. This means that the catalytic activity changes accordingto the crystallographic orientation of the copper surface, as shown in,for example, Y. Hori, I. Takahashi, O. Koga, N. Hoshi, “Electrochemicalreduction of carbon dioxide at various series of copper single crystalelectrodes”; Journal of Molecular Catalysis A: Chemical 199 (2003)39-47; or M. Gattrell, N. Gupta, A. Co, “A review of the aqueouselectrochemical reduction of CO₂ to hydrocarbons at copper”; Journal ofElectroanalytical Chemistry 594 (2006) 1-19.

In order to be able to provide all these crystallographic surfaces for ahigh efficiency of ethylene formation at high current density, theelectrode must not consist of a smooth sheet, but should be micro- tonanostructured.

The accessibility of such catalytically active sites limits theformation of ethylene to a Faraday efficiency of about 20%, or restrictsthe achievable current density to +/−10 mA/cm², as described in K. P.Kuhl, E. R. Cave, D. N. Abram, and T. F. Jaramillo, Energy andEnvironmental Science 5, 7050-7059 (2012).

Furthermore, H. Yano, T. Tanaka, M. Nakayama, K. Ogura, “Selectiveelectrochemical reduction of CO₂ to ethylene at a three-phase interfaceon copper(I) halide-confined Cu-mesh electrodes in acidic solutions ofpotassium halides”; Journal of Electroanalytical Chemistry 565 (2004)287-293, achieved current densities in the region of 100 mA/cm², butethylene here was enriched to a Faraday efficiency (FE) of about 80% inthe method of circulating the gaseous substances, and so the “intrinsic”Faraday efficiency of the electrode cannot be determined.

In summary, the current densities of methods known from the prior artare well below the values of relevance for economic utilization.

For the electrochemical reduction of CO₂ to ethylene, it was possiblewith the aid of copper catalysts deposited in situ to achieve currentdensities of 170 mA/cm² with a Faraday efficiency of >55% over anelectrolysis time of 60 min, as shown in in-house studies. However, inthe case of electrodes produced in this way, the selectivity of theelectrode can decrease with time, which can lead to an increase inhydrogen production. A change in the selectivity with time can becorrelated to structural coarsening of the material, which was alsoobservable, for example, from microscope images. Nano-dendritic copperstructures containing both Cu⁰ and Cu^(I) in the form of Cu₂O wereidentified as a selective catalyst.

Current densities of industrial relevance can be achieved using gasdiffusion electrodes (GDE). This is known from the existing prior art,for example, for chloralkali electrolyses operated on the industrialscale. The use of copper-based gas diffusion electrodes in electrolysiscells seems to be advantageous for an energy-efficient conversion ofmatter from CO₂ to hydrocarbons. One electrode-specific feature ofparticular interest is the selectivity (Faraday efficiency in %) and theconversion of matter (current density in mA/cm²).

Silver/silver oxide/PTFE (polytetrafluoroethylene)-based gas diffusionelectrodes have been used on the industrial scale in recent times forthe production of sodium hydroxide solution in existing chloralkalielectrolysis processes (oxygen-depolarized electrodes). It was possibleto increase the efficiency of the chloralkali electrolysis process by30-40% by comparison with conventional electrodes. The methodology ofcatalyst embedding with PTFE is known from a multitude of publicationsand patterns.

The known embedding methods are divided into three different processroutes:

-   -   1. Wet methods using a surfactant-stabilized PTFE microemulsion.    -   2. Wet methods using a surfactant-stabilized Nafion®        microemulsion.    -   3. Dry methods by calendering of premixed catalyst/PTFE        mixtures.

In this context, said wet method 1. can have the disadvantages mentionedhereinafter, aside from the fact that examples of gas diffusionelectrodes known from the literature contain the catalyst only as anadditive and consist mainly of bound conductive charcoal (for highconversions the catalyst loading should be high):

The suspensions or pastes that are usually applied by spraying or barcoating generally have long drying times, which means that continuousproduction with relatively large electrode areas (of industrialrelevance) is not economically possible. Excessively rapid drying leadsto cracking, called “mud cracking”, within the layers applied, whichmakes the electrode unusable.

The porosity of the layer applied is determined (generated) in thewet-chemical method virtually exclusively by the evaporation of thesolvent. This process is highly solvent- or boiling point-dependent andcan lead to a high reject rate of the electrodes produced, since theevaporation cannot be assured in a homogeneous manner over the entirearea. A further central disadvantage is the use of surface-activesubstances (surfactants) or thickeners, plasticizers, which are used forstabilization of the particle suspensions since they cannot be removedwithout residue by the corresponding drying phases or the thermalcrosslinking process.

The embedding process 2., wherein Nafion® (perfluorosulfonic acid, PFSA)is used as binder rather than PTFE, likewise has correspondingdisadvantages, since a wet-chemical method using appropriate surfactantsis being employed here too. Nafion® itself is a hydrophilic ionomerhaving highly acidic R—HSO₃ groups which can lead to unwanted acidcorrosion or partial dissolution of the metal in the case of somecatalysts. Nafion®-bound layers additionally have much lower porositythan PTFE-bound layers. The purely hydrophilic properties of Nafion® canlikewise be disadvantageous, since Nafion®, owing to its hydrophilicproperties, is unsuitable for formation of hydrophobic channels that areadvantageous for gas transport within a gas diffusion electrode. Usableelectrodes comprising Nafion® should therefore consist of multiplelayers in order to be able to implement the essential properties of aGDE. However, multilayer coating processes are not very attractive foreconomic reasons. Nafion®-based coating processes can additionally leadto unwanted formation of hydrogen.

The drying method 3. is based on a roll calendering process, for exampleof PTFE/catalyst powder. The corresponding technique can be traced backto EP 0297377 A2, according to which electrodes based on Mn₂O₃ wereproduced for batteries. DE 3710168A1 makes the first reference to theemployment of the drying process with regard to the preparation ofmetallic electrocatalyst electrodes. The technique was additionally usedin patents relating to the production of silver-based (silver(I) orsilver(II) oxide) gas diffusion electrodes (oxygen-depolarizedelectrodes). The patents EP 2444526 A2 and DE 10 2005 023615 A1 mentionmixtures having a binder content of 0.5-7%. The carrier used was Ag ornickel meshes having a wire diameter of 0.1-0.3 mm and a mesh size of0.2-1.2 mm. The powder is applied directly to the mesh before it issupplied to the roll calender. DE 10148599 A1 or EP 0115845 B1 describeda similar process in which the powder mixture is first extruded to givea sheet or film which is pressed onto the mesh in a further step.

Owing to the low mechanical stability, the latter method is lesssuitable than the above-specified one-step process. EP 2410079 A2describes the one-stage process for production of a silver-basedoxygen-depolarized electrode with the addition of metal oxidesupplements such as TiO₂, Fe₃O₄, Fe₂O₃, NiO₂, Y₂O₃, Mn₂O₃, Mn₅O₈, WO₃,CeO₂ and spinels such as CoAl₂O₄, Co(AlCr)₂O₄ and inverse spinels suchas (Co,Ni,Zn)₂(Ti,Al)O₄, perovskites such as LaNiO₃, ZnFe₂O₄.Supplements of silicon nitride, boron nitride, TiN, AlN, SiC, TiC, CrC,WC, Cr₃C₂, TiCN have likewise been found to be suitable, and oxides ofthe ZrO₂, WO₃ type have been identified as being particularly suitable.The materials are explicitly declared as fillers having no catalyticeffect. The aim here is explicitly the reduction of the hydrophobiccharacter of the electrode.

DE 10335184 A1 discloses catalysts which can be used as an alternativefor oxygen-depolarized electrodes: precious metals, e.g. Pt, Rh, Ir, Re,Pd, precious metal alloys, e.g. Pt—Ru, precious metal compounds, e.g.precious metal sulfides and oxides, and Chevrel phases, e.g. Mo₄Ru₂Se₈or Mo₄Ru₂S₈, where these may also contain Pt, Rh, Re, Pd etc.

Known Cu-based gas diffusion electrodes for generation of hydrocarbonson the basis of CO₂ are mentioned, for example, in the papers by R. Cook[J. Electrochem. Soc., vol. 137, no. 2, 1990]. This mentions awet-chemical method based on a PTFE 30B (suspension)/Cu(OAc)₂/Vulkan XC72 mixture. The method describes how a hydrophobic gas transport layeris applied using three coating cycles, and a catalyst-containing layerusing three further coatings. Each step is followed by a drying phase(325° C.) with a subsequent static pressing operation (1000-5000 psi).For the electrode obtained, a Faraday efficiency of >60% and a currentdensity of >400 mA/cm² are reported. However, reproduction experimentswhich are cited hereinafter as comparative examples demonstrate that thestatic pressing method described does not lead to stable electrodes. Theadded Vulkan XC 72 was likewise found to have an adverse effect, and soit was likewise not possible to obtain any hydrocarbons.

DE 101 30 441 A1 discloses a biporous pore system in a gas diffusionelectrode, but no two-layer structure. For such a one-layer structure,flooding of the electrode was observed in in-house preliminary tests. Aone-layer structure can also be found, for example, in DE 10 2010 031571 A1. According to DE 101 30 441 A1, a metallic support skeleton isrolled into a catalyst film produced in that document.

US 2013/0280625 A1 discloses a two-layer structure of a gas diffusionelectrode, but does not disclose any hydrophobic pores, and disclosesonly pores in the diffusion layer as hydrophilic layer. A sacrificialmaterial is used in an obligatory manner therein, and is required forformation of pores. However, in-house preliminary tests have shown thatthis is not appropriate to the aim.

SUMMARY OF THE INVENTION

There is thus a need for cathodes for carbon dioxide electrolysis, inwhich carbon dioxide can be converted effectively to hydrocarbons. Inaddition, it is an object of the invention to provide a catalyst conceptthat is not based on in situ copper deposition but provides a copper gasdiffusion electrode which can be processed to give an electrode.Moreover, it is an object of the present invention to develop selectiveelectro-catalysts having long-term stability and to embed them into gasdiffusion electrodes that can be connected to electrical contacts.

The inventors have found that particularly active and C₂H₄-selective gasdiffusion electrodes should satisfy a multitude of parameters requiredfor ethylene formation. There follows a discussion of propertiesspecific to the invention of an electrode of the invention. Furthermore,the inventors have found that specific demands on the catalyst arerequired in order that the electrode can form ethylene. These criteriaare not apparent from the prior art and constitute the basis for thedevelopment of hydrocarbon-selective electrodes of this kind.

The following important specific parameters and requirements for ahydrocarbon-selective gas diffusion electrode have been found:

-   -   Good wettability of the electrode surface in order that the        aqueous electrolyte or H⁺ ions can come into catalyst contact.        (H⁺ is required for ethylene or alcohols such as ethanol,        propanol or glycol.)    -   High electrical conductivity of the electrode or of the catalyst        and a homogeneous potential distribution across the entire        electrode area (potential-dependent product selectivity).    -   High chemical and mechanical stability in electrolysis operation        (suppression of cracking and corrosion).    -   Defined porosity with a suitable ratio between hydrophilic and        hydrophobic channels or pores (assurance of availability of CO₂        with simultaneous presence of H⁺ ions).

These can be achieved or fulfilled in accordance with the invention.

In a first aspect, the present invention relates to a gas diffusionelectrode comprising a preferably copper-containing carrier, preferablyin the form of a sheetlike structure, and a first layer comprising atleast copper and at least one binder, wherein the first layer compriseshydrophilic and hydrophobic pores and/or channels, further comprising asecond layer comprising copper and at least one binder, wherein thesecond layer is present atop the carrier and the first layer atop thesecond layer, wherein the content of binder in the first layer is lessthan in the second layer.

In a further aspect, the present invention relates to a process forproducing a gas diffusion electrode, comprising

-   -   producing a first mixture comprising at least copper and        optionally at least one binder,    -   producing a second mixture comprising at least copper and at        least one binder,    -   applying the second mixture comprising at least copper and at        least one binder to a preferably copper-containing carrier,        preferably in the form of a sheetlike structure,    -   applying the first mixture comprising at least copper and        optionally at least one binder to the second mixture,    -   optionally applying further mixtures to the first mixture, and    -   dry rolling the first and second mixtures and optionally further        mixtures onto the carrier to form a second layer and a first        layer and optionally further layers,        wherein the proportion of binder in the second mixture is 3-30%        by weight, preferably 10-30% by weight, further preferably        10-20% by weight, based on the second mixture, and wherein the        proportion of binder in the first mixture is 0-10% by weight,        preferably 0.1-10% by weight, further preferably 1-10% by        weight, even further preferably 1-7% by weight, even further        preferably 3-7% by weight, based on the first mixture, wherein        the content of binder in the first mixture is smaller than in        the second mixture; or comprising    -   producing a first mixture comprising at least copper and at        least one binder,    -   applying the first mixture comprising at least copper and        optionally at least one binder to a preferably copper-containing        carrier, preferably in the form of a sheetlike structure, and    -   dry rolling the first mixture onto the carrier to form a first        layer,        wherein the proportion of binder in the mixture is 3-30% by        weight, preferably 3-20% by weight, further preferably 3-10% by        weight, based on the first mixture.

The present invention additionally relates, in yet a further aspect, toan electrolysis cell comprising the gas diffusion electrode of theinvention.

Further aspects of the present invention can be taken from the dependentclaims and the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended drawings are intended to illustrate embodiments of thepresent invention and impart further understanding thereof. Inconjunction with the description, they serve to elucidate concepts andprinciples of the invention. Other embodiments and many of theadvantages mentioned are apparent with regard to the drawings.

The elements of the drawings are not necessarily shown to scale withrespect to one another. Elements, features and components that areidentical, have the same function and the same effect are each given thesame reference numeral in the figures of the drawings, unless statedotherwise.

FIG. 1 shows a schematic diagram of a gas diffusion electrode of theinvention with hydrophobic and hydrophilic regions or channels.

FIG. 2 shows a schematic diagram of production of a gas diffusionelectrode of the invention based on an illustrative PTFE-bound catalyst.

FIG. 3 shows a schematic of a further embodiment of a gas diffusionelectrode of the invention in the form of a multilayer preparation.

FIGS. 4 to 6 show, in schematic form, illustrative diagrams of apossible construction of an electrolysis cell in one embodiment of thepresent invention.

FIGS. 7 and 8 show illustrative configuration forms for a gasdistribution chamber downstream of a gas diffusion electrode of theinvention in an electrolysis cell of the invention.

FIG. 9 shows the results of Faraday efficiencies of the electrolysiscell from comparative example 3.

FIGS. 10 and 11 show the results of Faraday efficiencies of theelectrolysis cell from comparative example 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTIONDefinitions

“Hydrophobic” in the context of the present invention is understood tomean water-repellent. According to the invention, hydrophobic poresand/or channels are those that repel water. More particularly,hydrophobic properties are associated in accordance with the inventionwith substances or molecules having nonpolar groups.

“Hydrophilic”, by contrast, is understood to mean the ability tointeract with water and other polar substances.

In the application, figures are reported in % by weight, unless statedotherwise or apparent from the description.

In a first aspect, the present invention relates to

a gas diffusion electrode comprisinga preferably copper-containing carrier, preferably in the form of asheetlike structure, anda first layer comprising at least copper and at least one binder,wherein the (first) layer comprises hydrophilic and hydrophobic poresand/or channels, further comprising a second layer comprising copper andat least one binder, wherein the second layer is present atop thecarrier and the first layer atop the second layer, wherein the contentof binder in the first layer is less than in the second layer.

The second layer, just like the first layer, may comprise hydrophilicand/or hydrophobic pores and/or channels.

What is also described is a gas diffusion electrode comprising apreferably copper-containing carrier, preferably in the form of asheetlike structure, and

a first layer comprising at least copper and at least one binder,wherein the layer comprises hydrophilic and hydrophobic pores and/orchannels.

FIG. 1 illustrates the relations between hydrophilic and hydrophobicregions of a GDE, which can achieve a good triphasicliquid/solid/gaseous relationship. In this case, in the electrode, thereare hydrophobic channels or regions 1 and hydrophilic channels orregions 2 on the electrolyte side, with catalyst sites 3 of low activitypresent in the hydrophilic regions 2. In addition, there are inactivecatalyst sites 5 on the gas side.

Particularly active catalyst sites 4 are in the triphasicliquid/solid/gaseous region. An ideal GDE thus has maximum penetrationof the bulk material by hydrophilic and hydrophobic channels in order toobtain a maximum number of triphasic regions for active catalyst sites.In this respect, it should be ensured in accordance with the inventionthat the first layer comprises hydrophilic and hydrophobic pores and/orchannels. By suitable adjustment of the first layer, it is possible toachieve the effect that a maximum number of active catalyst sites arepresent in the gas diffusion electrode, which is explained further inthe further, especially preferred embodiments and/or the dependentclaims.

For hydrocarbon-selective gas diffusion electrodes for carbon dioxidereduction, accordingly, more intrinsic properties are needed than areoffered by the known systems. The electrocatalyst and the electrode areaccordingly in a close relationship.

The carrier here is not particularly restricted, provided that it issuitable for a gas diffusion electrode and preferably contains copper.For example, it is also possible for parallel wires to form a carrier inthe extreme case. In particular embodiments, the carrier is a sheetlikestructure, further preferably a mesh, very preferably a copper mesh.This can assure both adequate mechanical stability and functionality asa gas diffusion electrode, for example with regard to a high electricalconductivity. In particular embodiments, the carrier may also besuitable with regard to the electrical conductivity of the first layer.Through the use of copper in the carrier, it is possible to provide asuitable conductivity and to reduce the risk of inward entrainment ofunwanted extraneous metals. In preferred embodiments, the carriertherefore consists of copper. A preferred copper-containing carrier, inparticular embodiments, is a copper mesh having a mesh size w of 0.3mm<w<2.0 mm, preferably 0.5 mm<w<1.4 mm, and a wire diameter x of 0.05mm<x<0.5 mm, preferably 0.1 mm≤x≤0.25 mm.

In addition, by virtue of the fact that the first layer comprisescopper, it is also possible to assure a high electrical conductivity ofthe catalyst and, especially in conjunction with a copper mesh, ahomogeneous potential distribution across the entire electrode area(potential-dependent product selectivity).

In preferred embodiments, a preferably copper-containing mesh,preferably the copper mesh which is used as carrier, has a mesh size ofthe carrier between 0.3 and 2.0 mm, preferably between 0.5-1.4 mm, inorder to achieve good conductivity and stability.

In particular embodiments, the binder comprises a polymer, for example ahydrophilic and/or hydrophobic polymer, for example a hydrophobicpolymer, especially PTFE. This can achieve suitable adjustment of thehydrophobic pores or channels. More particularly, the first layer isproduced using PTFE particles having a particle diameter between 5 and95 μm, preferably between 8 and 70 μm. Suitable PTFE powders include,for example, Dyneon® TF 9205 and Dyneon TF 1750. Suitable binderparticles, for example PTFE particles, may, for example, be virtuallyspherical, for example spherical, and may be produced, for example, byemulsion polymerization. In particular embodiments, the binder particlesare free of surface-active substances. The particle size can bedetermined here, for example, according to ISO 13321 or D4894-98a andmay correspond, for example, to the manufacturer data (e.g. TF 9205:mean particle size 8 μm according to ISO 13321; TF 1750: mean particlesize 25 μm according to ASTM D4894-98a).

In addition, the first layer comprises at least copper which may, forexample, be in the form of metallic copper and/or copper oxide and whichfunctions as catalyst site.

In particular embodiments, the first layer comprises metallic copper inthe 0 oxidation state.

In particular embodiments, the first layer comprises copper oxide,especially Cu₂O. The oxide here may contribute to stabilizing the +1oxidation states of copper and hence to maintaining the selectivity forethylene with long-term stability. Under electrolysis conditions, it canbe reduced to copper.

In particular embodiments, the first layer comprises at least 40 at %(atom percent), preferably at least 50 at % and further preferably atleast 60 at % of copper, based on the layer. This can assure bothsuitable mechanical stability and suitable catalytic activity of thisfirst layer that serves as catalyst layer (CL). In particularembodiments, the copper for production of the gas diffusion electrode ofthe invention is provided as particles, which are defined furtherhereinafter.

In addition, the first layer may also comprise further promoters whichimprove the catalytic activity of the GDE in association with thecopper. In particular embodiments, the first layer comprises at leastone metal oxide preferably having a lower reduction potential than theevolution of ethylene, preferably ZrO₂, Al₂O₃, CeO₂, Ce₂O₃, ZnO₂, MgO;and/or at least one copper-rich intermetallic phase, preferably at leastone Cu-rich phase selected from the group of the binary systems Cu—Al,Cu—Zr, Cu—Y, Cu—Hf, CuCe, Cu—Mg and the ternary systems Cu—Y—Al,Cu—Hf—Al, Cu—Zr—Al, Cu—Al—Mg, Cu—Al—Ce with copper contents >60 at %;and/or copper-containing perovskites and/or defect perovskites and/orperovskite-related compounds, preferably YBa₂Cu₃O_(7-δ) where 0≤δ≤1(corresponding to YBa₂Cu₃O_(7-δ)X_(δ)), CaCu₃Ti₄O₁₂,La_(1.85)Sr_(0.15)CuO_(3.930)Cl_(0.053), (La,Sr)₂CuO₄.

Preferred promoters here are the metal oxides.

In particular embodiments, the metal oxide used is water-insoluble, inorder that aqueous electrolytes can be used in an electrolysis using thegas diffusion electrode of the invention. Moreover, by virtue of theredox potential of the metal oxide being lower than that of theevolution of ethylene, it is possible to ensure that ethylene can beprepared from CO₂ by means of the GDE of the invention. In particularembodiments, the oxides are not to be reduced either in a carbon dioxidereduction. Nickel and iron, for example, are unsuitable since hydrogenforms here. Moreover, the metal oxides are preferably not inert, butshould preferably constitute hydrophilic reaction sites that can servefor the provision of protons.

The promoters, especially the metal oxide, are able here to promote thefunction and production of electro-catalysts having long-term stability,in that they stabilize catalytically active copper nanostructures. Thestructural promoters here can reduce the high surface mobilities of thecopper nanostructures and hence reduce their tendency to sinter. Theconcept originates from heterogeneous catalysis and is used successfullywithin high-temperature processes.

Promoters used for the electrochemical reduction of CO₂ may especiallybe the following metal oxides that cannot be reduced to metals withinthe electrochemical window: ZrO₂ (E=−2.3 V), Al₂O₃ (E=−2.4 V), CeO₂(E=−2.3 V), MgO (E=−2.5). It should be noted here that the oxidesmentioned are not added as additives but are part of the catalystitself. The oxide, as well as its function as a promoter, also fulfillsthe feature of stabilizing copper in the I oxidation state andadditionally also intermediates in the reduction of carbon dioxide, suchas CO, C₂H₄ (or OH). There exist many Cu(I) complexes of CO and C₂H₄,which suggests stability of these postulated intermediates (see, forexample, H. Tropsch, W. J. Mattox, J. Am. Chem Soc. 1935, 57, 1102-1103;T. Ogura, Inorg. Chem., 1976, 15 (9), 2301-2303; J. S. Thompson, R. L.Harlow, J. F. Whitney, J. Am. Chem. Soc., 1983, 105 3522-3527; and V. A.K. Adiraju, J. A. Flores, M. Yousufuddin, H. V. Rasika Dias,Organometallics, 2012, 31, 7926-7932).

In particular embodiments, the catalyst has the following inventivefeatures: by contrast with the known heterogeneous Cu/Al₂O₃, Cu/ZrO₂,Cu/MgO/Al₂O₃ catalysts used in industry, in particular embodiments,preferably only very copper-rich catalysts having a molar proportionof >60 at % Cu are used for the electrochemical reduction of CO₂ owingto the electrical conductivity required.

Especially preferred in gas diffusion electrodes of the invention aremetal oxide/copper catalyst structures that are produced as follows:

For the production of the metal oxides, the precipitation, in particularembodiments, cannot be effected as frequently described in a pH regimebetween pH=5.5-6.5, but can be effected within a range between 8.0-8.5,such that the precursors formed are not hydroxide carbonates similar tomalachite (Cu₂[(OH)₂|CO₃]), azurite (Cu₃(CO₃)₂(OH)₂) or aurichalcite(Zn,Cu)₅[(OH)₆|(CO₃)₂)], but are hydrotalcites (Cu₆Al₂CO₃(OH)₁₆.4(H₂O)),which can be obtained in a greater yield. Likewise suitable are layereddouble hydroxides (LDHs) having a composition [M^(z+) _(1-x)M³⁺ _(x)(OH)₂]^(q+)(X^(n−))_(q/n).yH₂O where M^(l+)=Li⁺, Na⁺, K⁺, M²⁺=Ca²⁺,Mg²⁺, Cu²⁺ and M³⁺=Al, Y, Ti, Hf, Ga. The corresponding precursors canbe precipitated under pH control by co-dosage of a metal salt solutionand a basic carbonate solution. A particular feature of these materialsis the presence of particularly fine copper crystallites having a sizeof 4-10 nm, which are structurally stabilized by the oxide present.

It is possible to achieve the following effects: the metal oxide, owingto its high specific surface area, can lead to better distribution ofthe catalyst metal; highly dispersed metal sites can be stabilized bythe metal oxide; CO₂ chemisorption can be improved by the metal oxide;copper oxides can be stabilized.

The precipitation can be followed by drying with subsequent calcinationin an O₂/Ar gas stream. The oxide precursors produced, according to themethod, can also subsequently be reduced directly in an H₂/Ar gasstream, reducing solely the Cu₂O or CuO to Cu and conserving the oxidepromoter. The activation step can also be effected by electrochemicalmeans subsequently. In order to improve the electrical conductivity ofthe layer applied prior to the electrochemical activation, it is alsopossible to partly mix oxide precursors and activated precursors. Inorder to be able to increase the underlying conductivity, it is alsopossible to mix in 0-10% by weight of copper powder in a similarparticle size.

It is likewise not ruled out in accordance with the invention that theready-calendered electrode is subjected to a subsequentcalcination/thermal treatment before the electrochemical activation isconducted.

A further means of production of suitable electro-catalysts is based onthe approach of the production of copper-rich intermetallic phases, forexample Cu₅Zr, Cu₁₀Zr₇, Cu₅₁Zr₁₄, which can be prepared from the melt.Corresponding ingots can subsequently be ground and fully or partlycalcined in an O₂/argon gas stream and converted to the oxide form. Ofparticular interest are the Cu-rich phases of the binary systems Cu—Al,Cu—Zr, Cu—Y, Cu—Hf, CuCe, Cu—Mg and the corresponding ternary systemshaving Cu contents >60 at %: CuYAl, CuHfAl, CuZrAl, CuAlMg, CuAlCe.

Copper-rich phases are known, for example, from E. Kneller, Y. Khan, U.Gorres, The Alloy System Copper-Zirconium, Part I. Phase Diagram andStructural Relations, Zeitschrift für Metallkunde 77 (1), p. 43-48, 1986for Cu—Zr phases, from Braunovic, M.; Konchits, V. V.; Myshkin, N. K.:Electrical contacts, fundamentals, applications and technology; CRCPress 2007 for Cu—Al phases, from Petzoldt, F.; Bergmann, J. P.;Schurer, R.; Schneider, 2013, 67 Metall, 504-507 (see, for example,table 2) for Cu—Al phases, from Landolt-Bornstein—Group IV PhysicalChemistry Volume 5d, 1994, p. 1-8 for Cu—Ga phases, and from P. R.Subramanian, D. E. Laughlin, Bulletin of Alloy Phase Diagrams, 1988, 9,1, 51-56 for Cu—Hf phases.

TABLE 2 Copper-aluminum phases (taken from Petzoldt, F.; Bergmann, J.P.; Schürer, R.; Schneider, 2013, 67 Metall, 504-507) Cu Al HardnessSpec. el. Phase [% by wt.] [% by wt.] [HV] resistance [μΩcm] Cu 100 0100 1.75 Γ Cu₉Al₄ 80 20 1050 14.2 Δ Cu₃Al₂ 78 22 180 13.4 ζ₂ Cu₄Al₃ 7525 624 12.2 η₂ CuAl 70 30 648 11.4 Θ CuAl₂ 55 45 413 8.0 Al 0 100 60 2.9

In the case of these copper-rich intermetallic phases too, theproportion of copper is preferably greater than 40 at %, furtherpreferably greater than 50 at %, more preferably greater than 60 at %.

However, it is not ruled out here that the intermetallic phases alsocontain nonmetal elements such as oxygen, nitrogen, sulfur, seleniumand/or phosphorus, i.e. oxides, sulfides, selenides, nitrides and/orphosphides for example are present. In particular embodiments, theintermetallic phases have been partly oxidized.

In addition, it is possible to use the following copper-containingperovskite structures and/or defect perovskites and/orperovskite-related compounds for electro-catalysts, especially for theformation of hydrocarbons: YBa₂Cu₃O_(7-δ) where 0≤δ≤1, CaCu₃Ti₄O₁₂,La_(1.85)Sr_(0.15),CuO_(3.930)Cl_(0.053), (La,Sr)₂CuO₄. In addition, itis not ruled out that mixtures of these materials can be used forelectrode preparation or, as required, subsequent calcination oractivation steps are conducted.

In particular embodiments, the catalyst particles comprising orconsisting of copper, for example copper particles, which are used forproduction of the GDE of the invention, have a homogeneous particle sizebetween 5 and 80 μm, preferably 10 to 50 μm, further preferably between30 and 50 μm. In addition, the catalyst particles, in particularembodiments, have a high purity without traces of extraneous metal. Bysuitable structuring, optionally with the aid of promoters, it ispossible to achieve high selectivity and long-term stability.

It is likewise possible for the promoters, for example the metal oxides,to have a corresponding particle size in the production.

The above promoters can additionally achieve or improve the followingproperties:

-   -   Good wettability of the electrode surface in order that the        aqueous electrolyte or H⁺ ions can come into catalyst contact.        (H⁺ is required for ethylene or alcohols such as ethanol,        propanol or glycol.)    -   High chemical and mechanical stability in electrolysis operation        (suppression of cracking and corrosion).    -   Defined porosity with a suitable ratio between hydrophilic and        hydrophobic channels or pores (assurance of availability of CO₂        with simultaneous presence of H⁺ ions).

In order to further adjust the porosity of the electrode, in particularembodiments, it is possible to add copper powder supplements having aparticle diameter of 50 to 600 μm, preferably 100 to 450 μm, preferably100-200 μm. The particle diameter of these supplements, in particularembodiments, is ⅓- 1/10 of the total layer thickness of the layer.Rather than Cu, the supplement may also be an inert material such as ametal oxide. This can achieve improved formation of pores or channels.

A gas diffusion electrode of the invention can especially be produced bythe production process of the invention as described further down.

In particular embodiments, the first layer comprises less than 5% byweight of, further preferably less than 1% by weight of and even furtherpreferably no charcoal- and/or carbon black-based or -like fillers, forexample conductive fillers, based on the layer. It should be noted herethat methods known from the literature for GDE production generallyrefer, both for dry and wet application, to the addition of activatedcarbons, conductive blacks (such as Vulkan XC72), acetylene black orother charcoals. However, it has been found in accordance with theinvention that even traces of charcoals and/or carbon black candistinctly reduce the selectivity of the catalyst with respect tohydrocarbons and promote the unwanted formation of hydrogen.

Moreover, the first layer, in particular embodiments, does not containany surface-active substances. In particular embodiments, the firstand/or second layer additionally do not contain any sacrificialmaterial, for example a sacrificial material having a releasetemperature of roughly below 275° C., for example below 300° C. or below350° C., and especially any pore former(s) which can typically remain atleast partly in the electrode in the case of production of electrodesusing such a material.

It has been found by in-house experiments with electrodes produced bywet-chemical means that these residues irreversibly poison a Cu-basedcatalyst, and so electrodes thus produced did not show any CO₂ reductionto hydrocarbons. The use of surface-active substances or surfactants,for example Triton X, should therefore be avoided in particularembodiments, and so a wet-chemical procedure for the embedding ofCu-based catalysts is unsuitable.

If just one (first) layer is present in the GDE, the content orproportion of binder, for example PTFE, in particular embodiments, maybe 3-30% by weight, preferably 3-20% by weight, further preferably 3-10%by weight, even further preferably 3-7% by weight, based on the one(first) layer.

The GDE of the invention further comprises a second layer comprisingcopper and at least one binder, wherein the second layer is present atopthe carrier and the first layer atop the second layer, wherein thecontent of binder in the first layer is smaller than in the secondlayer. In addition, the second layer may comprise coarser copper orinert material particles, for example having particle diameters of 50 to700 μm, preferably 100-450 μm, in order to provide a suitable channel orpore structure.

In preferred embodiments, in this context, the second layer comprises3-30% by weight of binder, preferably 10-30% by weight of binder,further preferably 10-20% by weight of binder, preferably >10% by weightof binder, further preferably >10% by weight and up to 20% by weight ofbinder, based on the second layer, and the first layer preferablycomprises 0-10% by weight of binder, for example 0.1-10% by weight ofbinder, preferably 1-10% by weight of binder, further preferably 1-7% byweight of binder, even further preferably 3-7% by weight of binder,based on the first layer. The binder here may be the same binder as inthe first layer, for example PTFE. In addition, the particles forproduction of the second layer, in particular embodiments, maycorrespond to those in the first layer, but may also be differenttherefrom. The second layer here is a metal particle layer (MPL) beneaththe catalyst layer (CL). Through layering of this kind, it is possibleto specifically create highly hydrophobic regions in the MPL andgenerate a catalyst layer having hydrophilic properties. By virtue ofthe strongly hydrophobic character of the MPL, it is likewise possibleto prevent unwanted penetration of the electrolyte into the gastransport channels, i.e. flooding thereof. Moreover, the second layerforms the contact with the CO₂ and should therefore also be hydrophobic.

In particular embodiments, the second layer partly penetrates the firstlayer. This can be achieved, for example, by virtue of the process ofthe invention and enables a good transition between the layers withregard to diffusion.

As well as the second layer, the GDE of the invention may also havefurther layers, for example atop the first layer and/or on the otherside of the carrier.

For production of such a multilayer GDE, for example, it is firstpossible to apply, by sieving application, a mixture for an MPL based ona highly conductive copper mixture of dendritic copper having particlesizes between 5-100 μm, preferably less than 50 μm, and coarser copperor inert material particles having particle sizes of 100-450 μm,preferably 100-200 μm, having a PTFE content of 3-30% by weight,preferably 20% by weight, in a layer thickness of 0.5 mm for example, toa copper mesh having a mesh size of 1 mm for example (thickness, forexample, 0.2-0.6 mm, e.g. 0.4 mm), and to draw it down by means of aframe or coating bar. Corresponding dendritic copper may also be presentin the first layer. This may then be followed by further sievingapplication of the catalyst/PTFE mixture (CL), for example with a PTFEcontent of 0.1-10% by weight, and smoothing or drawdown, for example bymeans of a frame of thickness 1 mm, so as to obtain a total layerthickness (Hf) of 1 mm. The layer pre-prepared in this way can then befed to a calender having a gap width H₀=0.4-0.7 mm, preferably 0.5-0.6mm, and rolled out, so as to obtain a multilayer gas diffusion electrodeas shown schematically in FIG. 3, comprising a copper mesh 8, an MPL 9and a CL 10. The MPL can achieve better mechanical stability, a furtherreduction in the penetration of the electrolyte and better conductivity,especially when meshes are used as carriers.

Stepwise production of the GDE by respective sieving application androlling of each individual layer can lead to lower adhesion between thelayers and is therefore less preferred.

In a further aspect, the present invention relates to a process forproducing a gas diffusion electrode, comprising

-   -   producing a first mixture comprising at least copper and        optionally at least one binder,    -   producing a second mixture comprising at least copper and at        least one binder,    -   applying the second mixture comprising at least copper and at        least one binder to a preferably copper-containing carrier,        preferably in the form of a sheetlike structure,    -   applying the first mixture comprising at least copper and        optionally at least one binder to the second mixture,    -   optionally applying further mixtures to the first mixture, and    -   dry rolling the first and second mixtures and optionally further        mixtures onto the carrier to form a second layer and a first        layer and optionally further layers,        wherein the proportion of binder in the second mixture is 10-30%        by weight, preferably 10-20% by weight, based on the second        mixture, and wherein the proportion of binder in the first        mixture is 0-10% by weight, preferably 0.1-10% by weight,        further preferably 1-10% by weight, even further preferably 1-7%        by weight, even further preferably 3-7% by weight, based on the        first mixture, wherein the content of binder in the first        mixture is smaller than in the second mixture.

Also described is a process for producing a gas diffusion electrodecomprising

-   -   producing a first mixture comprising at least copper and at        least one binder,    -   applying the first mixture comprising at least copper and        optionally at least one binder to a preferably copper-containing        carrier, preferably in the form of a sheetlike structure, and    -   dry rolling the first mixture onto the carrier to form a first        layer,        wherein the proportion of binder in the mixture is 3-30% by        weight, preferably 3-20% by weight, further preferably 3-10% by        weight, even further preferably 3-7% by weight, based on the        first mixture.

The production of the first and second mixtures or of the first mixtureis not particularly restricted here and can be effected in a suitablemanner, for example by stirring, dispersing, etc.

When the second mixture is applied, the first mixture may also comprise0% by weight of binder, i.e. no binder, since binder from the secondmixture can diffuse into the first layer that forms from the firstmixture in the course of rolling and hence the first layer can also havea content of binder of, for example, at least 0.1% by weight, forexample 0.5% by weight, as established in preliminary experiments. Inparticular embodiments, however, the first mixture in the case ofapplication of 2 or more mixtures comprises binder.

In particular embodiments, the binder comprises a polymer, for example ahydrophilic and/or hydrophobic polymer, for example a hydrophobicpolymer, especially PTFE. This can achieve a suitable adjustment of thehydrophobic pores or channels. More particularly, for production of thefirst layer, PTFE particles having a particle diameter between 5 and 95μm, preferably between 8 and 70 μm, are used. Suitable PTFE powdersinclude, for example, Dyneon® TF 9205 and Dyneon® TF 1750.

In particular embodiments, the copper for the production of the mixtureis in the form of particles or catalyst particles, for example includingdendritic copper, having a homogeneous particle size between 5 and 80μm, preferably 10 to 50 μm, further preferably between 30 and 50 μm. Inaddition, the catalyst particles, in particular embodiments, have a highpurity without traces of extraneous metal. By suitable structuring,optionally with the aid of the promoters, as described above, it ispossible to achieve high selectivity and long-term stability.

By suitable adjustment of the particle sizes of copper and binder andany further additions such as promoters, it is possible to control thepores and/or channels, i.e. the hydrophobic and hydrophilic pores and/orchannels, of the GDE for the passage of gas and/or electrolyte and hencefor the catalytic reaction.

In particular embodiments, the first and/or second mixtures do notcontain any sacrificial material, for example a sacrificial materialhaving a release temperature of about below 275° C., for example below300° C. or below 350° C., and especially no pore former(s) which cantypically remain at least partly in the electrode in the case ofproduction of electrodes using such a material.

In particular embodiments, the first and/or second mixtures are notpasty, for example in the form of inks or pastes, but are in the form ofpowder mixtures.

The application of a first, second and further mixture(s) is notparticularly restricted and can be effected, for example, by scatteringapplication, sieving application, bar coating, etc.

The rolling application is likewise not particularly restricted and canbe effected in a suitable manner. Rolling of the mixture or mass(particles) into the structure of the carrier, for example a meshstructure, is explicitly desirable in particular embodiments in order toassure a high mechanical stability of the electrode.

By virtue of the aforementioned two-stage process with formation of afilm, this is not the case; the pre-extruded film here lies only on themesh and has lower adhesion, and also mechanical stability.

As a result, in the case of application of multiple layers too, it ispreferable that the mixtures for the layers are applied individually tothe carrier and are then rolled collectively, in order to achieve betteradhesion between the layers. In this way, the layers may at least partlypenetrate one another, for example in a thickness of 1-20 μm.

The mechanical stress on the binder, for example of polymer particles,by the rolling process leads to crosslinking of the powder through theformation of binder channels, for example PTFE fibrils. The attainmentof this state is particularly important in order to guarantee suitableporosity or mechanical stability of the electrode. The hydrophobicitycan be adjusted via the respective content of polymer or via thephysical properties of the catalyst powder. In the case of applicationof two (or more layers), a suitable binder content in the second mixturehas been found to be 10-30% by weight, preferably 10-20% by weight,based on the second mixture, and a suitable proportion of binder in thefirst mixture to be 0-10% by weight, 0.1-10% by weight, furtherpreferably 1-10% by weight, even further preferably 1-7% by weight, evenfurther preferably 3-7% by weight. In the case of application of justone mixture, a particularly suitable binder content, for example PTFEcontent, has been found to be 3-30% by weight, preferably 3-20% byweight, further preferably 3-10% by weight and even further preferably3-7% by weight of binder, based on the first mixture.

The degree of fibrillation of the binder, for example PTFE (structureparameter correlates directly with the shear rate applied, since thebinder, for example a polymer, behaves as a shear-thinning(pseudo-plastic) fluid in the rolling application. After the extrusion,the layer obtained, by virtue of the fibrillation, has an elasticcharacter. This change in structure is irreversible, and so this effectcannot be subsequently enhanced by further rolling; instead, the layer,by virtue of the elastic characteristics, is damaged with further actionof shear forces. Particularly significant fibrillation candisadvantageously lead to the electrode rolling up on the layer side,and so excessively high contents of binder should be avoided.

For dry rolling application, it is preferable that the water content inthe rolling operation corresponds, for example, to the ambient humidityat most. For example, the content of water and solvents in the rollingapplication is less than 5% by weight, preferably less than 1% by weightand, for example, even 0% by weight.

In particular embodiments, the copper-containing carrier is a coppermesh having a mesh size w of 0.3 mm<w<2.0 mm, preferably 0.5 mm<w<1.4mm, and a wire diameter x of 0.05 mm<x<0.5 mm, preferably 0.1 mm≤x≤0.25mm. The rolling into a mesh, for example a copper mesh, allows theinterstices in the mesh, for example copper mesh, to be effectivelybridged by the overlying (for example highly conductive) layer andenables complete 3D contact connection with the electrode. As a result,higher oxide contents are possible.

In particular embodiments, the production of the gas diffusion electrodeof the invention is additionally based on the exclusion of charcoal-and/or carbon black-based or -like fillers, for example conductivefillers. The catalyst itself or dendritic copper (formed, for example,through activation of the catalyst) or mixtures of the two serve here ascharcoal replacement.

In addition, the method of the invention, in particular embodiments,does not need any surface-active substances/surfactants or thickenersand additives (such as flow improvers) that have been identified ascatalyst poisons.

In particular embodiments, the bed height y of the first mixture on thecarrier in the application is in the range of 0.3 mm<y<2.0 mm,preferably 0.5 mm<y<1.0 mm. In the case of multiple layers, each layermay have a corresponding bed height y, but the bed heights of all layerspreferably do not add up to more than 2.0 mm, preferably to more than1.5 mm, more preferably to more than 1 mm.

In particular embodiments, the gap width in the rolling application H₀is the height of the carrier +40% to 50% of the total bed height Hf ofthe mixtures of the various layers, for example of the bed height y ofthe first mixture if it is the only one used.

In particular embodiments, the rolling application is effected by meansof a calender.

In particular embodiments, the copper content in the mixture is at least40 at %, preferably at least 50 at % and further preferably at least 60at % of copper, based on the mixture.

In particular embodiments, further additions to the mixture include:

at least one metal oxide having a lower reduction potential than theevolution of ethylene, preferably ZrO₂, Al₂O₃, CeO₂, Ce₂O₃, ZnO₂, MgO;and/or at least one copper-rich intermetallic phase, preferably at leastone Cu-rich phase selected from the group of the binary systems Cu—Al,Cu—Zr, Cu—Y, Cu—Hf, CuCe, Cu—Mg and/or the ternary systems Cu—Y—Al,Cu—Hf—Al, Cu—Zr—Al, Cu—Al—Mg, Cu—Al—Ce with copper contents >60 at %;and/or at least one metal for formation of a copper-rich metallic phase,preferably Al, Zr, Y, Hf, Ce, Mg, or at least two metals for formationof ternary phases, preferably Y—Al, Hf—Al, Zr—Al, Al—Mg, Al—Ce, suchthat the copper content is >60 at %;and/or copper-containing perovskites and/or defect perovskites and/orperovskite-related compounds, preferably YBa₂Cu₃O_(7-δ)X_(σ),CaCu₃Ti₄O₁₂, La_(1.85)Sr_(0.15)CuO_(3.930)Cl_(0.053), (La,Sr)₂CuO₄.

The addition of the metal for formation of a copper-rich metallic phase,preferably Al, Zr, Y, Hf, Ce, Mg, or at least two metals for formationof ternary phases, preferably Y—Al, Hf—Al, Zr—Al, Al—Mg, Al—Ce, suchthat the copper content is >60 at %, can be effected, for example, insuch a way that, in the production of the gas diffusion electrode,intermetallic phases are formed, for example through co-melting andthermal oxidation, and can then be selectively reduced, for example byelectrochemical means. However, such co-melting in the mixture iseffected here before the binder is added. In such a case, there is thusa sequence in that the metal is first added and fused with copper beforethe binder and any further substances are added to the mixture.

In particular embodiments, the process of the invention can thus beeffected by a calendering process as shown schematically in FIG. 2. Inthis case, the catalyst particles 6 and the binder particles 7, forexample PTFE particles, are rolled onto the carrier 8, here in the formof a copper mesh, with the aid of a calender 11.

In particular embodiments, the rolling or calendering is conducted at aroller speed between 0.3 and 3 rpm, preferably 0.5-2 rpm. In particularembodiments, the flow rate or an advance rate (of the GDE in length perunit time, for example in the case of calendering) Q is in the rangefrom 0.04 to 0.4 m/min, preferably 0.07 to 0.3 m/min.

In order to further adjust the porosity of the electrode, in particularembodiments, it is possible to add copper powder supplements having aparticle diameter of 50 to 600 μm, preferably 100 to 450 μm, furtherpreferably 100 to 200 μm, especially to the second mixture in the caseof application of multiple layers. The particle diameter of thesesupplements, in particular embodiments, is ⅓- 1/10 of the total layerthickness of the layer. Rather than copper, the supplement may also bean inert material such as a metal oxide. In this way, it is possible toachieve improved formation of pores or channels.

An illustrative process for producing a gas diffusion electrode may thusproceed, for example, as follows: the GDE can be produced using a drycalendering method in which a mixture of a cold-flowing polymer(preferably PTFE) and the respective pre-calcined catalyst powdercomprising copper and optionally a promoter is produced in an intensivemixing apparatus or laboratory scale with a knife mill (IKA). The mixingprocedure may, for example, follow the following procedure, but is notrestricted thereto: grinding/mixing for 30 sec and pause for 15 sec fora total of 6 min, these figures being based, for example, on the knifemill with a total loading of 50 g. After the mixing operation, the mixedpowder attains a slightly tacky consistency, with fibrillation here, forexample, of the binder, for example PTFE. According to the amount ofpowder or polymer/chain length chosen, there may also be variation inthe mixing time before this state is attained.

The powder mixture obtained is subsequently scattered or sieved onto acopper mesh having a mesh size of >0.5 mm and <1.0 mm and a wirediameter of 0.1-0.25 mm in a bed thickness of 1 mm. The powder mixtureapplied is then drawn down, for example, with a coating bar. Thisoperation can be repeated more than once until a homogeneous layer isobtained. Alternatively, the powder mixture can be pelletized during orafter the mixing operation in order to obtain a pourable material, forexample having an agglomerate diameter of 0.05 to 0.2 mm.

In order that the powder does not trickle through the mesh, the reverseside of the copper mesh can be sealed with a film subject to no furtherrestriction. The prepared layer is compacted with the aid of a two-rollrolling device (calender). The rolling process itself is characterizedin that a reservoir of material forms upstream of the roll. The speed ofthe roll is between 0.5-2 rpm and the gap width was adjusted to theheight of the carrier +40% to 50% of the bed height Hf of the powder, orcorresponds virtually to the thickness of the mesh +0.1-0.2 mm infeed.

In addition, the calender can also be heated. Preference is given totemperatures in the range of 20-200° C., preferably 20-50° C.

The catalyst itself can be processed prior to the application in thecalcined state, for example also as a metal oxide precursor, or alreadyin the reduced state. Mixtures of the two forms are possible. This isalso true in the case of the intermetallic phases or alloys described,and so these can likewise be used in the oxide form or in the metallicstate. Furthermore, it is not ruled out that the calendered electrodecan be calcined subsequently, for example at 300-360° C. for 5 to 15min.

It is advantageous for the gas diffusion electrode of the invention,especially in the case of hydrocarbon-selective copper catalystelectrodes, to apply a copper-PTFE base layer as a second layer forbetter contact connection with nanoscale materials, while simultaneouslymaintaining a high porosity. The base layer may be characterized by avery high conductivity, for example 7 mohm/cm or more, and preferablyhas a high porosity, for example of 50-70%, and a hydrophobic character.The binder content, for example PTFE, may be chosen, for example,between 3-30% by weight, for example 10-30% by weight. The intermediatecopper layer as the second layer may itself be catalytically active inthe region of the overlap zone with the catalyst layer as the firstlayer, and especially serves for better areal electrical connection ofthe electrocatalyst and can improve the availability of CO₂ owing to thehigh porosity. With the aid of this method, the required amount ofcatalyst can be reduced by a factor of 20-30. The correspondingelectrocatalyst/binder (e.g. PTFE) mixture can, in a first step, besieved out onto the reverse side of the current distributor andcalendered. It is additionally also possible to apply the 2-layervariant described as a double layer. The binder used, especially PTFE,in particular embodiments, should be treated beforehand in a knife millin order to achieve fiber formation. Particularly suitable PTFE powdershave been found to be, for example, Dyneon® TF 9205 and Dyneon® TF 1750.In order to promote this effect, abrasive hard materials may be mixed inthe range between 0-50% by weight. The following are examples ofsuitable materials: SiC, B₄C, Al₂O₃ (high-grade corundum), SiO₂ (crushedglass), preferably in a grain size of 50-150 μm. The production of thegas diffusion electrode with a binder-based (e.g. PTFE-based) diffusionbarrier is based on multiple layers that cannot be considered inisolation from one another, but preferably have an overlap zone ofmaximum breadth in the boundary regions, for example of 1-20 μm.

The method of two-layer construction additionally includes the option ofdispensing with binder materials as the first layer within the catalystlayer, which means that it is possible to achieve better electricalconductivity. It is likewise possible to process very ductile or brittlepowder particles. This is not possible in a single-layer construction.In the case of mechanically sensitive catalysts, it is possible todispense with the process step of the knife mill, which means that thecatalyst remains unchanged since mechanical stress resulting from themixing operation can be avoided.

Subsequent electrochemical activation of the electrode obtained, inparticular embodiments, can optionally be conducted, for example bychemical or electrochemical activation, and is not particularlyrestricted. An electrochemical activation procedure may lead topenetration of cations of the conductive salt of the electrolyte (e.g.KHCO₃, K₂SO₄, NaHCO₃, KBr, NaBr) into the hydrophobic GDE channels, thuscreating hydrophilic regions. This effect is particularly advantageousand has not been described to date in the literature.

In yet a further aspect, the present invention relates to anelectrolysis cell comprising a gas diffusion electrode of the invention,which is preferably used as cathode. In particular embodiments, the gasdiffusion electrodes of the invention can be operated specifically inplate electrolyzers.

The further constituents of the electrolysis cell, for instance theanode, optionally one or more membranes, inlet(s) and outlet(s), thevoltage source etc., and further optional devices such as cooling orheating units, are not particularly restricted in accordance with theinvention, nor are anolytes and/or catholytes that are used in such anelectrolysis cell, and the electrolysis cell, in particular embodiments,is used on the cathode side for reduction of carbon dioxide.

In the context of the invention, the configuration of the anode spaceand the cathode space is likewise not particularly restricted.

Illustrative configurations for an exemplary construction of a typicalelectrolysis cell and possible anode and cathode spaces are shown inFIGS. 4 to 6.

Electrochemical reduction of CO₂, for example, takes place in anelectrolysis cell typically consisting of an anode and a cathode space.FIGS. 4 to 6 which follow show examples of a possible cell arrangement.For each of these cell arrangements, it is possible to use a gasdiffusion electrode of the invention, for example as cathode.

By way of example, the cathode space II in FIG. 4 is configured suchthat a catholyte is supplied from the bottom and then leaves the cathodespace II at the top. Alternatively, the catholyte can also be suppliedfrom the top, as in the case, for example, of falling-film electrodes.At the anode A which is electrically connected to the cathode K by meansof a power source for provision of the voltage for the electrolysis, theoxidation of a substance which is supplied from the bottom, for example,with an anolyte takes place in the anode space I, and the anolytetogether with the oxidation product then leaves the anode space. In the3-chamber construction shown in FIG. 4, it is additionally possible toconvey a reaction gas, for example carbon dioxide, through the gasdiffusion electrode into the cathode space II for reduction. Althoughthey are not shown, embodiments with a porous anode are alternativelyconceivable. In FIG. 4, the spaces I and II are separated by a membraneM. By contrast, in the PEM (proton or ion exchange membrane)construction of FIG. 5, the gas diffusion electrode K and a porous anodeA directly adjoin the membrane M, by means of which the anode space I isseparated from the cathode space II. The construction in FIG. 6corresponds to a mixed form of the construction from FIG. 4 and theconstruction from FIG. 5, wherein a construction with the gas diffusionelectrode as shown in FIG. 4 is provided on the catholyte side, whereasa construction as in FIG. 5 is provided on the anolyte side. It will beappreciated that mixed forms or other configurations of the electrodespaces shown by way of example are also conceivable. Embodiments withouta membrane are additionally conceivable. In particular embodiments, theelectrolyte on the cathode side and the electrolyte on the anode sidemay thus be identical, and the electrolysis cell/electrolysis unit maynot need a membrane. However, it is not ruled out that the electrolysiscell in such embodiments has a membrane, although this is associatedwith additional cost and inconvenience with regard to the membrane andalso to the voltage applied. Catholyte and anolyte may also optionallybe mixed again outside the electrolysis cell.

FIGS. 4 to 6 are schematic diagrams. The electrolysis cells from FIGS. 4to 6 may also be combined to form mixed variants. For example, the anodespace may be configured as a PEM half-cell, as in FIG. 5, while thecathode space consists of a half-cell containing a certain electrolytevolume between membrane and electrode, as shown in FIG. 4. In particularembodiments, the distance between electrode and membrane is very smallor 0 when the membrane is porous and includes a feed of the electrolyte.The membrane may also have a multilayer configuration, such thatseparate feeds of anolyte and catholyte are enabled. Separation effectsare achieved in the case of aqueous electrolytes, for example, throughthe hydrophobicity of interlayers. Conductivity can nevertheless beassured if conductive groups are integrated into separation layers ofthis kind. The membrane may be an ion-conducting membrane, or aseparator that brings about mechanical separation only and is permeableto cations and anions.

The use of the gas diffusion electrode of the invention makes itpossible to construct a three-phase electrode. For example, a gas can beguided from behind toward the electrically active front side of theelectrode in order to conduct an electrochemical reaction there. Inparticular embodiments, there may also merely be flow along the back ofthe gas diffusion electrode, meaning that a gas such as CO₂ is guidedalong the back side of the gas diffusion electrode relative to theelectrolyte, in which case the gas can penetrate through the pores ofthe gas diffusion electrode and the product can be removed at the back.Preferably, the gas flow in the case of backflow is the reverse of theelectrolyte flow, in order that any liquid forced through can betransported away. In this case too, a gap between the gas diffusionelectrode and the membrane as electrolyte reservoir is advantageous.

By virtue of the sufficient porosity of the gas diffusion electrode, twomodes of operation are thus possible: one cell variant (a) enablesdirect active flow of a gas such as CO₂ through the GDE. The productsformed are removed from the electrolysis cell through the catholyteoutlet and separated from the liquid electrolyte in a downstream phaseseparator. A disadvantage of this method is the elevated mechanicalstress on the GDE and partial or complete forcing of the electrolyte outof the pores. Disadvantages are likewise found to be the elevatedoccurrence of gas in the electrolyte space and displacement of theelectrolyte. For the mode of operation, in addition, a high excess ofCO₂ is required. In particular embodiments, only gas diffusionelectrodes having a porosity >70% and elevated mechanical stability aresuitable for this mode of operation. The second cell variant describes amode of operation in which the CO₂ flows within the rear region of theGDE by virtue of an adjusted gas pressure. The gas pressure here shouldbe chosen such that it is equal to the hydrostatic pressure of theelectrolyte in the cell, such that no electrolyte is forced through. Anessential advantage of the cell variant is a higher conversion of thereaction gas used, for example CO₂, compared to the flow variant.

In order still to prevent passage of electrolyte through the gasdiffusion electrode, it is possible to apply a film on the side of thegas diffusion electrode remote from the electrolyte, i.e. on thecarrier, for example a mesh, in order to prevent the electrolyte frompassing through to the gas. The film here may be provided suitably andis hydrophobic for example.

In particular embodiments, the electrolysis cell has a membrane whichseparates the cathode space and the anode space of the electrolysis cellin order to prevent mixing of the electrolytes. The membrane here is notparticularly restricted, provided that it separates the cathode spaceand the anode space. More particularly, it essentially prevents passageof the gases that form at the cathode and/or anode through to the anodeor cathode space. A preferred membrane is an ion exchange membrane, forexample in polymer-based form. A preferred material for an ion exchangemembrane is a sulfonated tetrafluoroethylene polymer such as Nafion®,for example Nafion® 115. As well as polymer membranes, it is alsopossible to use ceramic membranes, for example the polymers that arementioned in EP 1685892 A1 and/or are laden with zirconia, for examplepolysulfones.

The material for the anode is likewise not particularly restricted anddepends primarily on the reaction desired. Illustrative anode materialsinclude platinum or platinum alloys, palladium or palladium alloys andglassy carbon. Further anode materials are also conductive oxides suchas doped or undoped TiO₂, indium tin oxide (ITO), fluorine-doped tinoxide (FTO), aluminum-doped zinc oxide (AZO), iridium oxide, etc.Optionally, these catalytically active compounds can also be appliedmerely superficially in thin-film methodology, for example on a titaniumcarrier.

The electrolysis cells from FIGS. 4 to 6 can also be combined to formmixed variants. For example, the anode space can be configured as aproton exchange membrane (PEM) half-cell, while the cathode spaceconsists of a half-cell containing a certain electrolyte volume betweenmembrane and electrode. In the ideal case, the distance betweenelectrode and membrane is very small or 0 when the membrane is porousand includes a feed of the electrolyte. The membrane may also have amultilayer configuration, such that separate feeds of anolyte andcatholyte are enabled. Separation effects are achieved in the case ofaqueous electrolytes, for example, through the hydrophobicity ofinterlayers. Conductivity can nevertheless be assured if conductivegroups are integrated into separation layers of this kind. The membranemay be an ion-conducting membrane, or a separator that brings aboutmechanical separation only.

For the distribution of a reaction gas, for example CO₂, behind a gasdiffusion electrode of the invention, i.e. on the carrier side, variousgas distribution chambers may be provided, of which two illustrative gasdistribution chambers are shown in FIGS. 7 and 8. These may be providedin order to further increase the residence time of a reaction gas suchas CO₂ and the associated conversion. The gas distributors, especiallyin the case of a gas diffusion electrode with backflow, can contributeto enhanced mass transfer across the entire electrode area.

Further aspects of the present invention relate to an electrolysissystem comprising an electrode of the invention or an electrolysis cellof the invention, and the use of the gas diffusion electrode of theinvention in an electrolysis cell or electrolysis system.

The further constituents of the electrolysis system are not restrictedany further and can be provided suitably.

The above embodiments, executions and developments can, if viable, becombined with one another as desired. Further possible configurations,developments and implementations of the invention also includecombinations that have not been mentioned explicitly of features of theinvention that have been described above or are described hereinafterwith regard to the working examples. More particularly, the personskilled in the art will also add individual aspects as improvements orsupplementations to the respective basic form of the present invention.

The invention is described hereinafter by some illustrative embodiments,but these do not restrict the invention.

EXAMPLES

All experiments and also the comparative examples and examples wereconducted at a room temperature of about 20° C.-25° C., unless statedotherwise.

The pressure in the comparative examples and examples was likewise notvaried, but left at room pressure (about 1.013 bar).

The further detailed data are reported for their respective comparativeexamples or examples.

COMPARATIVE EXAMPLES (NEGATIVE EXPERIMENTS) Comparative Example 1

In comparative example 1, a multilayer gas diffusion electrode wasproduced according to the instructions of R. Cook (J. Electrochem. Soc.1990, 137, 2).

The hydrophobic gas transport layer was produced according to thepublication:

2.5 g of Vulkan XC 72 and 2.8 g of Teflon 30B (DuPont) were dispersed in25 mL of water and applied to a dense copper mesh (100 mesh). The layerapplied was dried under air and compressed at 344 bar for 2 min. Thisprocedure was used to produce a total of three layers. This was followedby the compression application of three further catalyst-containinglayers having the following mixing ratio: 2.5 g of Vulkan XC 72, 2.61 gof Cu(OAc)₂*H₂O, 0.83 g of Teflon 30B, dispersed in 25 mL of H₂O. Eachlayer applied was dried under air and then compressed at 69 bar. Thefinished GDE was activated at 324° C. in a 10% by volume H₂/Ar gasmixture for 3-4 h and finally compressed once again at 69 bar for 30sec.

Result: No mechanically stable GDE was obtained over an area of 3.3 cm².The drying procedure led to unwanted “mud-cracking” of the layer.

Electrochemical characterization was accomplished using a test setupthat corresponds essentially to that of the above-described electrolysissystem from FIG. 6 with flow cells for electrolysis.

In the flow cell, the cathode used was the particular gas diffusionelectrode (GDE) with an active area of 3.3 cm², the gas feed rate ofcarbon dioxide on the cathode side was 50 mL/min, and the electrolyteflow rate on both sides was 130 mL/min. The anode was iridium oxide on atitanium carrier with an active area of 10 cm². The catholyte was a 1 MKHCO₃ solution with KHCO₃ in a 1 M concentration, and the anolyte was 1M KHCO₃, each in deionized water (18 MS)), each in an amount of 100 mL,and the temperature was 25° C. In addition, 0.5 M K₂SO₄ was also triedas catholyte, and 2.5 M KOH as anolyte.

In the electrochemical characterization of the GDE, it was not possibleto detect any ethylene, but exclusively hydrogen along with smallproportions of CO.

Comparative Example 2

In a further experiment, the water dispersant was exchanged for ethyleneglycol, and comparative example 2 otherwise corresponds to comparativeexample 1 unless stated otherwise. The use of the higher-boilingdispersant prevented cracking, but it was again not possible to detectany ethylene selectivity.

The following method was used for this purpose:

1.440 g of Vulkan XC 72 (49.5% by weight, 3.2 mg/cm²) were mixedvigorously with 15 mL of ethylene glycol with a disperser within 1 h.Then 2.44 g of a PTFE suspension (Teflon 30B, 50.41% by weight, 3.25mg/cm²) were added while stirring. The mixture was applied to a coppermesh that corresponded to the one used in comparative example 1 with acoating bar with a thickness of 100 μm and dried under air for at least24 h. Then the three further catalyst-containing layers were applied asin comparative example 1. Subsequently, the solvent was removed in adrying cabinet at 270° C. with a ramp of 10 K/min and isothermalconditions for 1 h. Thereafter, a layer corresponding to the first layerwas applied (thickness 100 μm) and the solvent was again removed asabove and left to dry under air for 24 h. The electrode was thencalcined in an oven at 350° C. with a ramp of 10 K/min and isothermalconditions for 2 h and compressed at 5 bar and 160° C. for 2 min.

Comparative Examples 3.1-3.5

The substrate used in comparative example 3.1 was a commerciallyavailable carbon cloth for gas diffusion electrodes (Flat® LT1400W,NuVant) in the form of a microporous layer.

A Nafion® D521 dispersion was applied to this gas diffusion layer aselectrocatalyst, which was produced as follows: 0.87 g of Cu(OAc)₂.H₂Owas dissolved in about 1 mL of H₂O. In addition, 1.36 g of Vulkan XC 72were mixed with 15 mL of ethylene glycol and the dissolved Cu(OAc)₂ wasadded and dispersed for 1 h. Thereafter, 1.5 g of the Nafion® D521suspension were added and stirred with a glass rod. Thereafter, themixture was applied to the hydrophobic gas diffusion layer, and driedunder air and then in a drying cabinet at 120° C. for 2 h. This wasfollowed by calcining in an oven at 250° C. with a slope of 10 K/min inan atmosphere of 10% by volume of H₂ in argon, and the calcining wascontinued under isothermal conditions for a total of 240 min.

The electrode thus obtained was subsequently characterized in terms ofits electrochemical properties with a test setup that, apart from theGDE, corresponded to the one from comparative example 1.

In this case, the copper catalyst was provided by reduction ofCu(OAc)₂.H₂O.

In the electrochemical characterization, the results shown in FIG. 9were achieved, which shows the Faraday efficiency as a function ofcurrent density. A Faraday efficiency of 10% is found for ethylene, butthis is not stable for a long period.

According to comparative example 3.1, the results shown in table 3 wereachieved by variation of the carrier (copper mesh with a mesh size of0.25 and a wire diameter of 0.14 mm) and of the mixture applied. Incomparative example 3.2, in addition, PTFE was used rather than Nafion®.

TABLE 3 Amounts and results in comparative examples 3.2-3.5 CarbonAmount of Max. FE binder Nafion ® Catalyst catalyst Catalyst for C₂H₄Carrier [% by wt.] [% by wt.] precursor [mg/cm² ] [% by wt.] [%] Cu mesh44.5 8.94 Cu (OAc)₂ 8.7 46.56 0.8% d = 0.14 (PTFE) [500 mA/cm²] Cu mesh44.2 16.8 Cu (OAc)₂ 14.2 39 1.6% d = 0.14 [400 mA/cm²] Elat ® 44.2 23.3Cu (OAc)₂ 14.2 39 3.8% LT1400W [400 mA/cm²] Elat ® 44 17.4 Cu (OAc)₂17.2 38.6 0.2% LT1400W [600 mA/cm²]

Comparative Examples 4.1-4.4

A multilayer gas diffusion electrode was produced as in comparativeexample 3.1, using a Cu/ZrO₂ catalyst that had been obtained from Cu₈Zr₃as catalyst. In comparative examples 4.2 and 4.4, the GDE wasadditionally reduced prior to the measurement, 4.3 relates to anelectrochemically activated electrode and 4.4 relates to ahydrogen-activated electrode. The amounts used and results obtained incomparative examples 4.1-4.4 are shown in table 4, with the resultsadditionally shown in FIGS. 10 and 11 for comparative example 4.3. Inthis context, FIG. 10 shows a current series, and FIG. 11 a measurementat constant current.

TABLE 4 Amounts and results in comparative examples 4.1-4.4 CarbonAmount of Max. FE binder Nafion ® Catalyst catalyst Catalyst for C₂H₄Carrier [% by wt.] [% by wt.] precursor [mg/cm²] [% by wt.] [%] Elat ®29.5 1.8 CuO/ZrO₂ 68.7 35.5 0.5% LT1400W [300 mA/cm²] Elat ® 29.5 1.8CuO/ZrO₂ 68.7 35.5 0.2% LT1400W [400 mA/cm²] Elat ® — 2.4 CuO/ZrO₂ 97.635.5 7.3% LT1400W [300 mA/cm²] Elat ® — 2.4 CuO/ZrO₂ 97.6 35.5 3.3%LT1400W [300 mA/cm²]

In the case of use of Cu/ZrO₂ as catalyst, a stable product spectrum wasobtained over an electrolysis time of 150 min.

In general, the charcoal-based GDEs in comparative examples 1-4 showedelevated Faraday efficiencies for hydrogen. It was concluded from thisthat carbon in the form of conductive blacks or activated carbons isless suitable for the production of ethylene-selective gas diffusionelectrodes.

Comparative Example 5

Subsequently, therefore, a GDE based on an aqueous PTFE dispersion withpure copper powder having a grain size of <45 μm was produced inaccordance with the method in Chemical Engineering and Processing 52(2012) 125-131. In this method, there was absolutely no use of carbon inthe form of conductive blacks or activated carbons.

The material used was the following:

PTFE suspension: TF5035R, 58% by weight (Dyneon™), Surfactant:Triton-100 (Fluka Chemie AG)

Thickener: hydroxyethyl methylcellulose (WalocelMKX 70000 PP 01, WolffCellulosics GmbH & Co. KG).

As the starting mixture, a solution that contained 97% by weight of Cuand 3% by weight of PTFE was produced as follows: 150 g of thickenersolution (1% by weight of methylcellulose in H₂O), 90.0 g of copperpowder, 53.7 g of H₂O and 1.5 g of surfactant were dispersed with anUltra-Turrax T25 disperser at 13 500 rpm for 5 min (wait for 2 min afterdispersing for 1 min).

Thereafter, 4.8 g of PTFE suspension were stirred in with a glass rodand the suspension obtained was applied to a copper mesh as used incomparative example 3.2 at 100° C.

A further GDE was produced on the basis of 0.5% by weight of PTFE by thesame procedure. The gas diffusion electrodes produced had very poorwettabilities and, in the case of the 0.5% PTFE content, poorporosities, as determined visually and by microscopy. In addition, itwas found that the GDEs contained considerable proportions of thesurfactant used, which was identified as a catalyst poison in acontrolled experiment. It was likewise not possible to drive out thecorresponding catalyst poison Triton X 100((p-tert-octyl-phenoxy)polyethoxyethanol) without residue attemperatures of >340° C., as confirmed by scanning electron microscopy.

Exclusively hydrogen was obtained with the electrodes produced by thisprocedure. The experiments made it clear that the use of surfactants isdisadvantageous for the formation of ethylene. The method likewise didnot lead to homogeneous porosities and, in the case of 3% by weight ofPTFE, led to very poor wettability.

Reference Example 1 Production of a Mixed Metal Oxide Catalyst byCoprecipitation:

Illustrative Method for Cu/Al₂O₃

An appropriate hydrotalcite precursor of the composition[Cu_(0.6)Al_(0.4)(OH)₂](CO₃)_(0.4).mH₂O (unknown water content for thefreshly precipitated hydrotalcite) is prepared by a coprecipitation.Simultaneously added are a 0.41 M metal salt solution (A) composed ofCu(NO₃)₂.3H₂O (0.246 mol) and Al(NO₃)₃.9H₂O (0.164 M) and ahydroxide/carbonate solution (B) composed of 0.3 M NaOH (12 g), 0.045 M(NH₄)₂CO₃ (4.32 g), such that the pH is between pH 8 and 8.5.

The addition rate of the metal salt solution was chosen as 120 mL/h.Oswalt ripening was effected for 30 min. Thereafter, the solids werefiltered off and washed to neutrality. Thereafter, the precursor wasdried at 80° C. for 12 h, pulverized and calcined. The calcination stepis effected in a tubular furnace having a temperature ramp of β=2 K/minup to 300° C. with isothermal conditions for 4 h in an argon/oxygenmixture: 20% by volume of O₂/Ar with a flow rate of 200 sccm. Theprecursor prepared was sieved before use.

Examples: Production of Powder-Based GDEs Comparative Example 6

A catalyst powder is prepared by coprecipitation of Cu(NO₃)₂.3H₂O andZrO(NO₃)₂.xH₂O according to reference example 1 with the respectivemolar amounts (mol). The pre-calcined catalyst powder (weight 45 g;particle size <75 μm by sieve analysis) is mixed on the laboratory scalewith a knife mill (IKA) (on a large scale, for example, with anintensive mixing apparatus) with PTFE particles (weight 5 g; Dyneon® TF1750; particle size (d50)=8 μm according to manufacturer). The mixingprocedure follows the following procedure: grinding/mixing for 30 secand wait for 15 sec for a total of 6 min. This statement is based on theknife mill with total loading 50 g. The mixed powder attains a slightlytacky consistency after the mixing operation. The mixing time beforethis state is attained may also vary according to the amount of powderor the polymer chosen or the chain length. The powder mixture obtainedis subsequently applied by scattering or sieving to a copper mesh havinga mesh size of >0.5 mm and <1.0 mm and a wire diameter of 0.1-0.25 mm ina bed thickness of 1 mm.

In order that the powder does not trickle through the mesh, the reverseside of the copper mesh can be sealed with a film subject to no furtherrestriction. The prepared layer is compacted with the aid of a two-rollrolling device (calender). The rolling process itself is characterizedin that a reservoir of material forms upstream of the roll. The speed ofthe roll is between 0.5-2 rpm and the gap width was adjusted to theheight of the carrier +40% to 50% of the bed height Hf of the powder, orcorresponds virtually to the thickness of the mesh +0.1-0.2 mm infeed.

The gas diffusion electrode obtained is activated in an electrolysisbath in a 1 M KHCO₃ solution at a current density of 15 mA/cm² for 6 h.

Comparative Example 7

Dendritic copper powder (45 g; particle size <45 μm, determined bysieving with appropriate mesh size (45 μm)) is mixed with 5 g of PTFE inan IKA knife mill by the procedure described in comparative example 6,and processed under the same conditions to give a GDE. After activation,the GDE described gave a Faraday efficiency of 16% at 170 mA/cm², whichremained constant over the measurement time of about 90 min.

Comparative Example 8

Cu₁₀Zr₇ is calcined in a tubular furnace with a temperature ramp of β=2K/min up to 600° C. with isothermal conditions for 4 h in anargon/oxygen mixture (20% by volume of O₂/Ar with a flow rate of 200sccm). The oxide precursor prepared, prior to use, is ground in aplanetary ball mill (Pulverisette) for 3 min and subsequently sieved(particle size <75 μm). 45 g of the catalyst obtained are mixed with 5 gof PTFE in an IKA knife mill by the procedure described in comparativeexample 6 and processed under the same conditions to give a GDE.

The GDEs from comparative examples 6 to 8 can be used in an electrolysiscell as described above or hereinafter, for example as cathode withwhich CO₂ can be reduced.

Example 1 Production of a 2-Layer Electrode

Copper powder with a particle diameter of 100-200 μm and PTFE TF 1750Dyneon were mixed in an IKA A10 knife mill for 6 min (grinding for 15sec, wait for 30 sec). The powder layer was then sieved off and gradedby means of a template of thickness 0.5 mm to form a base layer. Thiswas followed by extrusion with a 2-roll calender with a roll separationof 0.5 mm. Thereafter, a catalyst layer was applied by sievingapplication, for example in each case analogously to comparativeexamples 6 to 8, through a 0.2 mm frame, and extrusion was againeffected with a 2-roll calender with a roll separation of 0.35 mm. Theresult was a highly porous base layer with a porosity of >70%, goodmechanical stability and very good conductivity at 5 mohm/cm. It waspossible to use catalysts with a copper content of 40% by weight.

Preferably, the catalysts had a purity above the commercially availablematerials or quality standards, as in the example as well. This wasdetectable by means of (surface-sensitive) XPS. SEM/EDX mapping analyseslikewise did not indicate any impurities at all in the hydrophobic baselayer.

It was additionally found that a copper content of >70% is advantageousin order to enable a low electrical resistance of the catalyst. Theeffect of the binder (PTFE) content with respect to the carrier oxide ismuch smaller in terms of the effect on conductivity.

Illustrative construction of a typical electrolysis cell: Theelectrochemical reduction of the CO₂ takes place in an electrolysis cellwhich typically consists of an anode space and a cathode space. FIGS. 4to 6 show examples of a possible cell arrangement. The concept presentedhereinafter is applicable to each of these cell arrangements.

The electrolysis cells from FIGS. 4 to 6 can also be joined to formmixed variants. For example, the anode space can be executed as a protonexchange membrane (PEM) half-cell, while the cathode space consists of ahalf-cell containing a certain electrolyte volume between membrane andelectrode. In the ideal case, the distance between electrode andmembrane is very small or 0 when the membrane is porous and includes afeed of the electrolyte. The membrane may also have a multilayerconfiguration, such that separate feeds of anolyte and catholyte areenabled. Separation effects are achieved in the case of aqueouselectrolytes, for example, through the hydrophobicity of interlayers.Conductivity can nevertheless be assured if conductive groups areintegrated into separation layers of this kind. The membrane may be anion-conducting membrane, or a separator that brings about mechanicalseparation only.

The present invention provides the possibility of producingethylene-selective, dimensionally stable gas diffusion electrodes basedon catalyst powder. This technique constitutes the basis for theproduction of electrodes on a larger scale, which can achieve currentdensities of >170 mA/cm² according to the mode of operation. All themethods known to date for production of ethylene-selective copperelectrodes are unsuitable for scaleup or are not dimensionally stable.Gas diffusion electrodes of the invention, by contrast, can be obtainedby suitable adjustment of a rolling process, especially a calenderingprocess.

It is possible in accordance with the invention to obtain highlyelectrically conductive, especially metal oxide-stabilized, coppercatalysts with copper nanostructures that enable oxidation cyclingbetween Cu(I)/Cu(0).

In particular embodiments, the production of the gas diffusion electrodeof the invention is additionally based on the exclusion of conductivefillers based on charcoals or carbon blacks. The charcoal substituteused here is the catalyst itself or dendritic copper or mixtures of thetwo. Moreover, the method of the invention, in particular embodiments,does not need surface-active substances/surfactants or thickeners andadditives (such as flow improvers) which have been identified ascatalyst poisons.

What is claimed is:
 1. A gas diffusion electrode comprising: acopper-containing carrier, and a first layer comprising at least copperand at least one binder, the first layer comprising hydrophilic andhydrophobic pores, a second layer comprising copper and at least onebinder, the second layer present atop the carrier and the first layeratop the second layer, wherein the content of binder in the first layeris less than the binder in the second layer, and wherein the secondlayer comprises 3-30% by weight of binder, and the first layer comprises0-10% by weight of binder.
 2. The gas diffusion electrode as claimed inclaim 1, wherein the first layer does not comprise charcoal-based and/orcarbon black-based fillers.
 3. The gas diffusion electrode as claimed inclaim 1, wherein the first layer does not comprise surface-activesubstances.
 4. The gas diffusion electrode as claimed in claim 1,wherein the first layer comprises at least 40 at % of copper, based onthe layer.
 5. The gas diffusion electrode as claimed in claim 1, whereinthe copper-containing carrier is a copper mesh.
 6. The gas diffusionelectrode as claimed in claim 1, wherein the first layer comprises atleast one metal oxide having a lower reduction potential than theevolution of ethylene selected from the group consisting of ZrO₂, Al₂O₃,CeO₂, Ce₂O₃, ZnO₂, and MgO.
 7. The gas diffusion electrode as claimed inclaim 1, wherein the second layer partly penetrates the first layer. 8.A process for producing a gas diffusion electrode, comprising: producinga first mixture comprising at least copper and optionally at least onebinder, producing a second mixture comprising at least copper and atleast one binder, applying the second mixture comprising at least copperand at least one binder to a copper-containing carrier, in the form of asheetlike structure, applying the first mixture comprising at leastcopper and optionally at least one binder to the second mixture, and dryrolling the first and second mixtures onto the carrier to form at leastfirst and second layers, wherein the proportion of binder in the secondmixture is 3-30% by weight of binder, based on the second mixture, andwherein the proportion of binder in the first mixture is 0-10% byweight, based on the first mixture, where the content of binder in thefirst mixture is smaller than in the second mixture.
 9. The process asclaimed in claim 8, wherein the copper-containing carrier comprises acopper mesh having a mesh size w of 0.3 mm<w<2.0 mm and a wire diameterx of 0.05 mm<x<0.5 mm.
 10. The process as claimed in claim 8, whereinthe bed height y of the first mixture on the carrier is in the range of0.3 mm<y<2.0 mm.
 11. The process as claimed in claim 8, wherein the gapwidth in the rolling application H₀ is the height of the carrier +40% to50% of the total bed height Hf of the first mixture.
 12. The process asclaimed in claim 8, wherein the rolling is effected by a calender. 13.The process as claimed in claim 8, wherein the copper content in themixture is at least 40 at % of copper, based on the mixture.
 14. Theprocess as claimed in claim 8, wherein the mixture further comprises atleast one metal oxide having a lower reduction potential than theevolution of ethylene including at least one of ZrO₂, Al₂O₃, CeO₂,Ce₂O₃, ZnO₂, and MgO.
 15. An electrolysis cell comprising a gasdiffusion electrode as claimed in claim
 1. 16. The gas diffusionelectrode as claimed in claim 1, wherein the second layer comprises10-30% by weight binder and the first layer comprises 0.1-10% by weightbinder.
 17. The gas diffusion electrode as claimed in claim 1, whereinthe second layer comprises 10-20% by weight binder and the first layercomprises 1-10% by weight binder.
 18. The gas diffusion electrode asclaimed in claim 1, wherein the first layer comprises 1-7% by weightbinder.
 19. The gas diffusion electrode as claimed in claim 1, whereinthe first layer comprises 3-7% by weight binder.
 20. The gas diffusionelectrode as claimed in claim 1, wherein the first layer comprises atleast one copper-rich intermetallic phase selected from the group ofsystems consisting of Cu—Al, Cu—Zr, Cu—Y, Cu—Hf, CuCe, Cu—Mg, Cu—Y—Al,Cu—Hf—Al, Cu—Zr—Al, Cu—Al—Mg, Cu—Al—Ce with copper contents >60 at %.21. The gas diffusion electrode as claimed in claim 1, wherein the firstlayer comprises one or more of copper-containing perovskites, defectperovskites, and perovskite-related compounds.
 22. The gas diffusionelectrode as claimed in claim 1, wherein the first layer comprisesYBa₂Cu₃O₇ where 0≤δ≤1.
 23. The gas diffusion electrode as claimed inclaim 1, wherein the first layer comprises a compound selected from thegroup consisting of CaCu₃Ti₄O₁₂,La_(1.85)Sr_(0.15)CuO_(3.930)Cl_(0.053), and (La,Sr)₂CuO₄.
 24. The gasdiffusion electrode as claimed in claim 1, wherein the first layercomprises at least 50 at % of copper, based on the layer.
 25. The gasdiffusion electrode as claimed in claim 1, wherein the first layercomprises at least 60 at % of copper, based on the layer.
 26. Theprocess as claimed in claim 8, wherein the copper-containing carriercomprises a copper mesh having a mesh size w of 0.5 mm<w<1.0 mm and awire diameter x of 0.1 mm×x≤0.25 mm.
 27. The process as claimed in claim8, wherein the bed height y of the first mixture on the carrier is inthe range of 0.5 mm≤y≤1.0 mm.
 28. The process as claimed in claim 8,wherein the mixture further comprises at least one copper-richintermetallic phase selected from the group of the systems consisting ofCu—Al, Cu—Zr, Cu—Y, Cu—Hf, CuCe, Cu—Mg, Cu—Y—Al, Cu—Hf—Al, Cu—Zr—Al,Cu—Al—Mg, and Cu—Al—Ce with copper contents >60 at %.
 29. The process asclaimed in claim 8, wherein the mixture further comprises at least onemetal for formation of a copper-rich metallic phase selected from thegroup consisting of Al, Zr, Y, Hf, Ce, Mg, Y—Al, Hf—Al, Zr—Al, Al—Mg,and Al—Ce, such that the copper content is >60 at %.
 30. The process asclaimed in claim 8, wherein the mixture further comprises at least oneof copper-containing perovskites, defect perovskites, andperovskite-related compounds.
 31. The process as claimed in claim 8,wherein the mixture further comprises YBa₂Cu₃O_(7-δ) where 0≤δ≤1. 32.The process as claimed in claim 8, wherein the mixture further comprisesat least one of CaCu₃Ti₄O₁₂, La_(1.85)Sr_(0.15)CuO_(3.930)Cl_(0.053),and (La,Sr)₂CuO₄.